PROJECT SUMMARY Toxoplasma gondii is an intracellular protozoan parasite that poses a major threat to patients with HIV/AIDS. The replicative stage (tachyzoite) develops into a latent stage (bradyzoite) that resides inside host cells as cysts in brain, heart, and skeletal muscle tissues. Tissue cysts remain in the host for life, as they are impervious to the immune response and currently approved drugs, and give rise to recurrent reactivation of infection in immune compromised patients. Despite its clinical importance, little is known about the regulation of gene expression required for the tachyzoite to bradyzoite transition. Our lab has previously established that epigenetic modifications, namely the acetylation of histone proteins by GCN5 family lysine acetyltransferases (KATs), regulate gene expression changes required for bradyzoite differentiation. Toxoplasma possesses two GCN5 KATs, GCN5a and GCN5b. Previously, we have shown that GCN5b partners with a complex of proteins to regulate the expression of housekeeping genes and is essential for the viability of tachyzoites; in contrast, GCN5a is dispensable in tachyzoites but is required to upregulate genes associated with stress responses that induce bradyzoite development. Importantly, loss of GCN5a results in a failure to activate bradyzoite-specific genes BAG1 and LDH2 in bradyzoite culture conditions. Consequently, we hypothesize that GCN5a works in a multi-subunit complex that is critical for tachyzoite development into bradyzoites. The aforementioned studies were completed in type I RH parasites, and while this strain does not develop into mature bradyzoites at high frequency, it has served to support the idea that GCN5a activates genes in response to bradyzoite inducing agents. It is now imperative that we examine the function of GCN5a in type II parasites, which readily form mature tissue cysts in vitro and in vivo. In this R21 application, we propose to address our hypothesis with two specific aims: (i) We will determine the role of GCN5a in bradyzoite conversion by characterizing mutants made in cystogenic parasites. (ii) We will identify proteins that comprise the GCN5a KAT complex in both tachyzoite and bradyzoite growth conditions, which promises to reveal new players involved in tissue cyst formation. The study of GCN5a provides a unique opportunity to gain much-needed knowledge about the formation of latent tissue cysts. Completion of our specific aims will generate reagents and datasets that are required to understand the role of GCN5a in bradyzoite development and generate further hypotheses that will determine how Toxoplasma persists in HIV/AIDS patients.